1. Field of the Invention
This invention relates to the method and apparatus for transporting and compressing gases, particularly flammable gases such as natural gas, but also other gases such as air.
2. Prior Art
There is presently no widespread use of natural gas to fuel automobiles in the United States. One major problem with using natural gas for this purpose is that fueling facilities are expensive to construct on a commercial scale. Moreover, gas fuels occupy a large volume unless they are stored at high pressure. Compressing gas fuels to high pressures with conventional mechanically-driven or rotary gas compression equipment is relatively expensive. In addition, compressing gases to high pressures (such as 1500 psig or higher) tends to cause all but the most durable compressor materials to wear quickly.
Conventional compressors cannot operate over a wide range of inlet pressures (such as about 400-3000 psig) that is generally required for mobile gas compression or delivery systems. Inlet pressures for conventional compressors are generally limited to narrow ranges, based upon the working pressures of the equipment and the ratio of the outlet pressure to the inlet pressure for each stage (usually a ratio of 3-4 for each stage is the maximum operable ratio for conventional compressors). In addition, conventional compressors typically often have a low maximum inlet pressure capability (such as 200 psig) due to the pressure limitations of the compressor case and crankshaft seals. The above limitations inherent in conventional compressors limits the use of these compressors for mobile gas delivery.
In addition to the above, present mobile gas delivery systems are inefficient because (absent a compressor on the delivery system) they cannot empty their stored volume below the pressure of the gas recipient.
Despite the problems associated with natural gas as a vehicle fuel, there is nevertheless increased impetus for the use of such gas for automobiles and other vehicles. Natural gas is generally less expensive than other fuels (on an equivalent thermal unit basis), and it burns relatively cleanly, alleviating increasing environmental concerns. Thus improved gas compression and transportation equipment will be increasingly valuable.
Some alternatives to conventional compressor systems have been developed for compressing natural gas. U.S. Pat. No. 4,585,039 to Hamilton discloses a system wherein fuel gas at low pressure is supplied to an inlet at the top of an upright working cylinder. The working cylinder is then filled with liquid through a bottom liquid inlet to force the gas from the cylinder and direct it into a storage cylinder. A check valve prevents backflow of gas from the storage cylinder as the liquid is drained from the working cylinder and as the working cylinder is again filled with low pressure gas. The process of filling the working cylinder with liquid to force gas from it into the storage cylinder is repeated to fill a gas storage cylinder. Two working cylinders may be provided so that, as one of them is drained, the other is filled with liquid. In this manner gas may be forced into a storage cylinder until the desired high pressure is achieved. Like the working cylinders, the storage cylinder has gas forced from it by filling it with liquid.
The system described in the Hamilton patent uses an open reservoir liquid hydraulic system as a liquid control means for alternately introducing liquid into the working cylinders and thereby forcing gas from these working cylinders. As shown in Hamilton, a system of electronic relays, pressure switches, and solenoid valves are used to control the liquid as it is switched from cylinder to cylinder. The liquid is pumped from an open reservoir directly into each cylinder as the control system dictates.
The Hamilton system pumps liquid from about atmospheric pressure to the discharge pressure of the gas in the cylinders. Upon switching of the solenoid valves, the liquid in the cylinders discharges from gas discharge pressure to about atmospheric pressure. In systems of this type, if the discharge pressure is sufficiently high, a significant vibration caused by decompression may be observed which is caused by the large pressure drop as the liquid is drained to atmospheric pressure.
The system shown in Hamilton requires that the fluid be pumped from essentially atmospheric pressure to the discharge pressure of the gas being compressed. The large pressure differential also generally causes energy transmitted to the liquid to be released in the form of heat when the liquid is returned to atmospheric pressure. Energy losses to heat may represent more than 50% of the total energy input to the system when compressing gas from about 1500 psig to about 3000 psig. The resultant heat will require a heat exchanger to handle peak heat dissipation loads. This exchanger load may sometimes approach 80% of the prime mover horsepower, which generally requires an addition of a large and expensive heat exchanger to the system. The energy loss through this exchanger represents needless and inefficient system energy consumption.
U.S. Pat. No. 4,515,516 to Perrine et al. discloses a two-cylinder gas compression system similar to the Hamilton system. In one embodiment of the Perrine et al. system, a closed loop hydraulic system is utilized. Instead of using an open liquid reservoir and unidirectional pump (with the accompanying valves for liquid flow switching), a reversible pump with a motor in a closed hydraulic system is used. Thus, one input-output of the pump is connected directly to the line leading to one cylinder, while the other input-output is connected directly to another line leading to the other cylinder.
In the Perrine et al. closed loop hydraulic system, the pump moves liquid into one cylinder until the divider reaches a desired position at the top of that cylinder. At this point a magnetic or pressure sensor changes state causing the pump to change direction and pump liquid from that cylinder into the other cylinder. This process is repeated for each cylinder, causing the pump to again reverse and pump liquid to the other cylinder upon the direction of the sensor. The Perrine et al. patent discloses use of a variable volume pump.
An advantage of the Perrine et al. closed loop hydraulic system is that the hydraulic fluid is not released to atmospheric pressure after each compression cycle. Thus vibration effects are generally reduced because the pressure differential is reduced between: (1) the inlet and outlet of the pump, and (2) the inlet and outlet of the liquid sent to the compression cylinders. Moreover, energy efficiency of the system is generally increased for the same reason.
Both U.S. Pat. No. 4,515,516 to Perrine et al. and No. 4,585,039 to Hamilton are incorporated by reference.